The present invention relates to a heat resistant security document containing at least one composition which activates to produce a color upon the application of a solvent, heat or abrasion. More particularly, the invention relates to a security document having an insulating material blended with such an activatable composition which allows the document to be passed through heated printing devices such as laser printers without the problem of premature color development. The composition(s) may be used to provide authentication and/or security features on a document.
Security documents are printed on a wide variety of commercial printing devices. Traditional mechanical impact printers have been typically been used in the past for printing information on security documents such as checks. The inks used with most impact printers adhere well to a document due to the partial penetration of the ink into the surface of the document substrate. In addition, dyes or other coatings may be applied to the documents to aid in the prevention of fraudulent alteration of security documents. For example, a solvent sensitive coating comprising a mixture of a color former and a color developer may be used to produce covert (i.e., initially colorless) images on a security document. If attempted alteration of such a document is made by the application of solvents, the color former and/or color developer dissolve and react to form a visible image on the substrate, providing easy detection of the attempted alteration.
However, with the advance of microcomputer technology, a number of faster printing methods have been developed to take advantage of the high-speed printing output which is now possible. Laser printers are an example of nonimpact printers which are faster, quieter, and more reliable than impact printers. Such printers operate by fusing toner images onto a substrate by passing the substrate through a pair of rolls which apply both heat and pressure. However, such printers have significant limitations which have prevented their wide use in printing documents such as checks and other security documents. One limitation is the inability to achieve satisfactory toner bonding on a large variety of paper products used to make such documents. Because of the lack of strong adherence of toner to paper, documents printed using such printers may be subject to deliberate alteration by counterfeiters, forgers, and the like.
Toner adhesion-enhancing coatings have been developed which improve the adhesion of toner to the documents. For example, U.S. Pat. No. 5,045,426 to Maierson et al describes a polymeric toner adhesion-enhancing composition for use on documents printed with noncontact printing devices. However, documents containing such toner adhesion-enhancing coatings may still be subject to alteration by other methods as the solvent sensitive coatings used in the art as described above cannot withstand passage through a laser printer. This is due to the high temperature of the toner fuser rollers which will melt the dye and/or developer and cause premature color development on the documents.
Accordingly, there is still a need in the art for a security document which may be printed using a high-speed printing device such as a laser printer without the problem of premature coloration and which has security features which can provide authentication of the document and/or indicate attempted alterations of the document.